Nearly five months after the release of PHP 5.3.0, the PHP developers have released the first maintenance update to the 5.3 branch of their popular programming language. The PHP 5.3.1 update focuses on stability and includes approximately 100 bug fixes, some of which are security related.
The latest release features the addition of the "max_file_uploads" INI directive, which can be used to limit the number of file uploads for each request to 20 by default. By limiting the number of uploads per-request, users can prevent possible denial of service (DoS) attacks. Missing sanity checks around EXIF (exchangeable image file format) processing have also been added. The developers advise all users to upgrade to the latest release.

LinuxSecurity is making available the release notes here:

PHP 5.3.1 Release Announcement

The PHP development team is proud to announce the immediate release of PHP 5.3.1. This is a maintenance release in the 5.3 series, which includes a large number of bug fixes.

Security Enhancements and Fixes in PHP 5.3.1:

Added "max_file_uploads" INI directive, which can be set to limit the number of file uploads per-request to 20 by default, to prevent possible DOS via temporary file exhaustion.

Added missing sanity checks around exif processing.

Fixed a safe_mode bypass in tempnam().

Fixed a open_basedir bypass in posix_mkfifo().

Fixed bug #50063 (safe_mode_include_dir fails).

Fixed bug #44683 (popen crashes when an invalid mode is passed).

Key Bug Fixes in PHP 5.3.1 include:

Fixed crash in com_print_typeinfo when an invalid typelib is given.

Fixed crash in SQLiteDatabase::ArrayQuery() and SQLiteDatabase::SingleQuery() when calling using Reflection.

Fixed crash when instantiating PDORow and PDOStatement through Reflection.

Fixed bug #49910 (no support for ././@LongLink for long filenames in phar tar support).

Fixed bug #49908 (throwing exception in __autoload crashes when interface is not defined).

Around 100 other bug fixes

For users upgrading from PHP 5.2 there is a migration guide available here, detailing the changes between those releases and PHP 5.3.